Raymond M. Brusasco

Environmental applications of low-temperature plasmas

Treatment of NOx in diesel engine exhaust represents a big opportunity for the environmental application of low-temperature plasmas. This paper discusses the effect of gas composition on the NOx conversion chemistry in a plasma. It is shown that the plasma by itself cannot chemically reduce NOx to N2 in the highly oxidizing environment of a diesel engine exhaust. To implement the reduction of NOx to N2, it is necessary to combine the plasma with a heterogeneous process that can chemically reduce NO2 to N2. Data is presented that demonstrates how the selective partial oxidation of NO to NO2 in a plasma can be utilized to enhance the selective reduction of NOx to N2 by a catalyst.

Plasma-assisted catalytic reduction of NOx

Many studies suggest that lean-NOx SCR proceeds via oxidation of NO to NO¬ by oxygen, followed by the reaction of the NO¬ with hydrocarbons. On catalysts that are not very effective in catalyzing the equilibration of NO+O¬ and NO¬, the rate of N¬ formation is substantially higher when the input NOx is NO¬ instead of NO. The apparent bifunctional mechanism in the SCR of NOx has prompted the use of mechanically mixed catalyst components, in which one component is used to accelerate the oxidation of NO to NO¬, and another component catalyzes the reaction between NO¬ and the hydrocarbon. Catalysts that previously were regarded as inactive for NOx reduction could therefore become efficient when mixed with an oxidation catalyst. Preconverting NO to NO¬ opens the opportunity for a wider range of SCR catalysts and perhaps improves the durability of these catalysts. This paper describes the use of a non-thermal plasma as an efficient means for selective partial oxidation of NO to NO¬. When combined with some types of SCR catalyst, the plasma can greatly enhance the NOx reduction and eliminate some of the deficiencies encountered in an entirely catalyst-based approach. efficiency for reduction of NOx

Feasibility of plasma aftertreatment for simultaneous control of NOx and particulates

Plasma reactors can be operated as a particulate trap or as a NO{sub x} converter. The soluble organic fraction (SOF) of the trapped particulates can be utilized for the oxidation of NO to NO{sub 2}. The NO{sub 2} can then be used to non-thermally oxidize the carbon fraction of the particulates. This paper examines the energy density required for oxidation of the SOF hydrocarbons and the fate of NO{sub 2} during the oxidation of the particulate carbon. The energy density required for complete oxidation of the SOF hydrocarbons is shown to be unacceptably large. The reaction of NO{sub 2} with carbon is shown to lead mainly to backconversion of NO{sub 2} to NO. These results suggest that the use of a catalyst in combination with the plasma will be required to efficiently reduce the NO{sub x} and oxidize the SOF hydrocarbons.

Localized CO2-laser treatment for mitigation of 351-nm damage growth in fused silica

A technique for inhibiting the growth of laser-induced surface damage on fused silica, initiated and propagated at the 351-nm laser wavelength, has been investigated. The technique exposes the damage sites to single pulses of a CO2 laser operating at the 10.6 micrometers wavelength at or near beam focus. This method results in a very localized treatment of the laser damage site and modifies the site such that laser damage does not propagate further. A laser damage site initiated with a single pulse of 355-nm laser light at approximately 45 J cm-2 and 7.5-ns pulse duration grows rapidly upon further illumination at 8 J cm-2 with 100% probability. Treatment of these sites with single pulses of 10.6 micrometers laser light for one second at a power level of between 17 and 37 Watts with a beam diameter of 5 mm alters the damage site such that it does not grow with subsequent 351-nm laser illumination at 8 J cm-2 10-ns pulse duration for > 1000 shots. The technique has been found to be 100% effective at stopping the growth of the laser damage.

Methods for mitigating surface damage growth in NIF final optics

We report a summary of the surface damage, growth mitigation effort at 3(omega) for fused silica optics at LLNL. The objective was to experimentally validate selected methods that could be applied to pre-initiated or retrieved-from- service optics, to stop further damage growth. A specific goal was to obtain sufficient data and information of successful methods for fused silica optics to select a single approach for processing NIF optics. This paper includes the test results and the evaluation thereof, for several mitigation methods for fused silica. The mitigation methods tested in this study are wet chemical etching, cold plasma etching, CO2 laser processing, and micro-flame torch processing. We found that CO2 laser processing produces the most significant and consistent results to halt laser-induced surface damage growth on fused silica. We recorded successful mitigation of the growth of laser-induced surface damage sites as large as 0.5-mm diameter, for 1000 shots at fluences in the range of 8 to 13 J/cm2. We obtained sufficient data for elimination of damage growth using CO2 laser processing on sub-aperture representative optics, to proceed with application to full- scale NIF optics.

CO2-laser polishing for reductoin of 351-nm surface damage initiation in fused silica

We have applied a carbon dioxide (CO2) raster scanning laser polishing technique on two types of fused silica flat optics to determine the efficacy of CO2-laser polishing as a method to increase the 351-nm laser damage resistance of optic surfaces. R-on-1 damage test results show that the fluence for any given 355-nm damage probability is 10-15 J/cm2 higher (at 3 ns pulse length, scaled) for the CO2-laser polished samples. Poor quality and good quality surfaces respond to the treatment such that their surface damage resistance is brought to approximately the same level. Surface stress and the resultant effect on wavefront quality remain key technology issues that would need to be addressed for a robust deployment.

UV-laser conditioning for reduction of 351-nm damage initiation in fused silica

This paper describes the effect of 355-nm laser conditioning on the concentration of UV-laser-induced surface damage sites on large-aperture fused silica optics. We will show the effect of various 355-nm laser conditioning methodologies on the reduction of surface-damage initiation in fused silica samples that have varying qualities of polishing. With the best, generally available fused silica optic, we have demonstrated that 355-nm laser conditioning can achieve up to 10x reduction in surface damage initiation concentration in the fluence range of 10-14 J/cm2 (355- nm at 3 ns).

Sulfur Tolerance of Selective Partial Oxidation of NO to NO2 in a Plasma

Several catalytic aftertreatment technologies rely on the conversion of NO to NO2 to achieve efficient reduction of NOx and particulates in diesel exhaust. These technologies include the use of selective catalytic reduction of NOx with hydrocarbons, NOx adsorption, and continuously regenerated particulate trapping. These technologies require low sulfur fuel because the catalyst component that is active in converting NO to NO2 is also active in converting SO2 to SO3 . The SO3 leads t o increase in particulates and/or poison active sites on the catalyst. A non-thermal plasma can be used for the selective partial oxidation of NO to NO2 in the gas-phase under diesel engine exhaust conditions. This paper discusses how a non-thermal plasma can efficiently oxidize NO to NO2 without oxidizing SO2 to SO3 .

Methods for mitigating growth of laser-initiated surface damage on DKDP optics at 351 nm

We report an experimental investigation of mitigating surface damage growth at 351 nm for machine-finished DKDP optics. The objective was to determine which methods could be applied to pre-initiated or retrieved-from-service optics, in order to stop further damage growth for large aperture DKDP optics used in high-peak-power laser applications. The test results, and the evaluation thereof, are presented for several mitigation methods applied to DKDP surface damage. The mitigation methods tested were CW-CO2 laser processing, aqueous wet-etching, short-pulse laser ablation, and micro-machining. We found that micro-machining, using a single crystal diamond tool to completely remove the damage pit, produces the most consistent results to halt the growth of surface damage on DKDP. We obtained the successful mitigation of laser-initiated surface damage sites as large as 0.14 mm diameter, for up to 1000 shots at 351 nm and fluences in the range of 2 to 13 J/cm2, ≈ 11 ns pulse length. Data obtained to-date indicates that micro-machining is the preferred method to process large-aperture optics.

Radio frequency and microwave plasma for optical thin-film deposition

For the next generation of fusion lasers reflecting mirrors with laser damage thresholds of at least 40 J/cm2 for 1 0 ns laser pulses at 1 .064 pm are needed. Up to now, no deposition technique has been developed to produce such mirrors. Best R&D-values realized today are around 30 J/cm2 for e-beam evaporated mirrors. R&D on conventional e-beam coating processes over the last 1 0 years has come up with marginal improvements in laser damage thresholds only. However, new technologies, like PICVD (Plasma-Impulse CVD) developed for the fabrication of ultra-low loss fiber preforms, seem to offer the potential to solve this problem. First results have been reported already [1-3]. It is well known that fused silica produced by CVD processes can have laser damage thresholds as high as 80 J/cm2. However, the thickness of a single deposited film is in the pm-range for most of the CVD-processes used for preform manufacturing; since interference optics need films in the ; /4n range (where n is the refractive index of the dielectric material) the use of preform-fabrication processes for the purpose of interference mirror fabrication is limited to a few plasma based CVD technologies, namely PCVD (Plasma-CVD, Philips [4]; PICVD, SCHOTT [5]). Especially PICVD is a very powerful technology to fabricate thin film multilayers for interference mirrors, because this technique is able to produce films down to monolayer thickness with nearly perfect stoichiometry and morphology. In first and preliminary experiments the usual deposition in a circular tube at high temperatures has been used for simplicity. However, to produce large area high quality laser mirrors this principle know-how has to be transfered from circular to planar geometry. Experiments showed, that there may be some limitations with respect to the homogeneity of a planar deposition using microwave excitation for the plasma. Therefore experiments have been performed in parallel with both RF and microwave excitation for comparison. In the following we will restrict ourselves to the description and discussions of the planar processes; the principle and details of the PICVD-process are described elsewhere [5] while RF-plasma technology is a well known process.